In high-speed reproduction machines, such as electrostatographic copiers and printers, a photoconductive member (or photoreceptor) is charged to a uniform potential and then a light image of an original document is exposed onto a photoconductive surface by a digital image driven laser. Exposing the charged photoreceptor to a light image discharges the photoconductive surface in areas corresponding to non-image areas in the original document while maintaining the charge on the image areas to produce an electrostatic latent image of the original document on the photoconductive surface of the photoreceptor. A developer material is then brought into contact with the surface of the photoconductive member to transform the latent image into a visible reproduction. The developer material includes toner particles with an electrical polarity opposite that of the photoconductive member, causing them to be attracted to the image on the photoconductive member. A blank print substrate, such as a sheet of paper, is brought into contact with the photoconductive member and the toner materials are transferred to it by electrostatic charging of the substrate. The substrate is subsequently heated and pressed to bond the reproduced image to the substrate permanently to produce a hard print reproduction of the original document or image. Thereafter, the photoconductive member is cleaned and reused for subsequent print production.
Various sizes of print substrates are typically stored in trays that are mounted at the side of the machine. In order to duplicate a document, a print substrate with the appropriate dimensions is transported from the appropriate tray into the paper path just ahead of the photoreceptor. The substrate is then brought into contact with the toner image on the surface of the photoconductive member prior to transfer. However, a registration mechanism typically intercepts the substrate in advance of the photoconductive member and either stops it or slows it down in order to synchronize the substrate with the image on the photoconductive member. The registration mechanism also properly aligns the print substrate in the process or longitudinal direction prior to delivery of the substrate to the photoconductive member. The registration mechanism also properly aligns the print substrate in the cross-process or lateral direction prior to delivery of the substrate to the photoconductive member.
The process of transferring charged toner particles from an image bearing member, such as the photoreceptive member, to an image support substrate, such as a print sheet, is accomplished at a transfer station. In a conventional electrostatographic machine, transfer is achieved by transporting an image support substrate into the area of the transfer station where electrostatic force fields sufficient to overcome the forces holding the toner particles to the photoconductive surface are applied to the substrate to attract and transfer the toner particles to the image support substrate. In general, such electrostatic force fields are generated by an electrostatic induction device, such as a corotron. The reverse side of the print sheet is exposed to a corona discharge while the front of the print sheet is placed in direct contact with the developed toner image on the photoconductive surface. The corona discharge generates ions having a polarity opposite that of the toner particles, thereby electrostatically attracting and transferring the toner particles from the photoreceptive image bearing member to the print sheet.
Unfortunately, the interface between the image bearing surface and the print sheet is not always optimal. Particularly, with non-flat print sheets, such as print sheets that have already passed through a fixing operation (e.g., heat or pressure fusing), perforated sheets, or sheets that are brought into imperfect contact with the charge retentive surface, the contact between the sheet and the image bearing surface may be non-uniform, which produces gaps where physical contact fails. The toner particles tend not to transfer across these gaps, causing a print quality defect referred to as transfer deletion.
The problem of transfer deletion has been addressed by various approaches. For example, mechanical devices that force the substrate into intimate and complete contact with the image bearing surface have been incorporated into transfer systems. Using this approach, transfer assist blades (TABs) have been configured for sweeping over the back side of the substrate at the entrance to the transfer region. The pressure applied by a TAB helps release toner from the image bearing surface to the substrate by holding the substrate flat in the electrostatic field. The transfer assist blade is typically moved from a non-operative position spaced from the substrate to an operative position where the TAB contacts the back of the substrate. A mechanism supporting the TAB is operable to press the TAB against the substrate with a pre-determined force sufficient to press the copy substrate into contact with the developed image on the photoconductive or other charged imaging surface. This pressure substantially eliminates any spaces between the substrate and the photoconductive member during the transfer process.
Control of the TAB movement is an important aspect of the image transfer operation. In printing systems in which the transfer substrates are cut sheets, no portion of the transfer assist blade should contact the photoreceptive member surface. Such contact may result in the pickup of residual dirt and toner from the charged photoconductive member surface to the transfer assist blade. Additionally, contact of the TAB with the charged photoreceptive member surface risks abrading the surface, thereby adversely affecting subsequent image quality and shortening the expected life of the expensive photoconductive member or other charged imaging surface. The spaces on the photoconductive member between images are known as inter-document zones (IDZs). Frequently, test patterns or other indicia are printed in these areas to evaluate the operational status of the components generating the images in the printing system and these test patterns are not transferred to the sheets. Thus, the TAB needs to move between the non-operative and operative positions in a manner that corresponds to the length of the images being transferred to the substrate without contacting the photoreceptive member in the IDZs.
Because the TAB is raised and lowered at the trailing edge and at the leading edge of the images, respectively, the configuration of the actuator and drive train moving the TAB is important. A high degree of accuracy is therefore required in timing engagement and disengagement of the TAB with the substrate. Such engagements and disengagements of the TAB are generally designed as timed sequences in relation to the substrate path speed and other related parameters. Some of the drive trains utilize cams that are rotated by the actuator to press the TAB into contact with the substrate. Other drive trains are configured with gears or links that are maneuvered by the actuator to move the TAB. Each different type of drive train has advantages for different print job parameters. For example, some provide quick TAB take-off from a trailing edge, while others apply TAB pressure sooner or more quickly than others.
The effects of these differences can be easily seen by the graph of the transfer functions shown in FIG. 9. This figure shows how four TAB drive trains respond differently to the same motor pulses due to differences in the drive train. The table shown below the graph quantifies the different responses. As shown in the figure, some drive trains enable the TAB to rotate into contact with the substrate quickly, while others lift the TAB from the substrate faster than the others. Likewise, some of the drive trains apply the full pressure of the TAB faster than other drive trains. These differences in drive train response to the actuator result in some drive trains being better for some types of substrates than for others. Incorporating all of the drive train configurations in a single printer and then selectively applying the drive trains to the TAB with reference to the substrate type is too expensive and requires additional space for the TAB mechanism.
Some printing parameters affect the effectiveness of the TAB on the transfer process. For example, printed images that have a substantial amount of marking material that will be placed near the leading edge of a sheet may be susceptible to smear, if the TAB lands at a rate or force that disturbs the sheet as the TAB encounters the sheet. To compensate for this issue, previously known systems delay the time that the TAB engages the sheet. While this adjustment may improve the transfer of the marking material near the leading edge, it may still result in sub-optimal transfer of the marking material since the sheet has no holding force against the sheet prior to the TAB touchdown. Additionally, this adjustment may not perform well with subsequent images that do not have as much marking material near the leading edge Similar issues occur at the trailing edge with regard to the position at which the TAB lifts from the sheet. While providing a plurality of drive trains in a printer and selectively coupling each one to the TAB to obtain the advantages of each one for particular print jobs might solve this issue, significant space would be required in the printer. Additionally, maintenance of so many drive trains would affect the reliability of the printer. Enabling TAB control to assist the transfer process more consistently over a wide range of print job parameters would be useful.